1. Field of the Invention
This invention relates to an ion sensor and, more particularly, to an ion sensor exhibiting a high speed of response, an excellent temperature response and stable operation.
2. Description of the Prior Art
An ion sensor most commonly used in the prior art is constituted by a glass membrane electrode. However, the glass membrane in such an electrode breaks easily, and frequent washing is necessary since measurements taken thereby are influenced by the existence of interfering ions, medicines, the proteins in a living body, amino acids and trace amounts of active ingredients. These and other inconveniences are encountered in terms of use.
PH electrodes using a polymeric membrane have recently been reported as an improvement on the glass membrane electrode. For example, a liquid membrane electrode is described in Analytical Chimica Acta, 131, (1981), pp. 111-116, and a solid membrane electrode is disclosed in Japanese Patent Application No. 59-281076. However, since these electrodes possess an internal liquid chamber just as the glass membrane electrode, they do not fundamentally solve the problems of the glass electrode and are not fully satisfactory.
These conventionally employed ion sensors generally can be used for measurements at a constant temperature. A change, especially a rapid one, in the temperature of a liquid specimen has a profound effect upon the sensor characteristics (speed of response, transient response, etc.). It is common technical knowledge that measurement of ionic concentration using an electrode having an internal liquid chamber is performed under constant temperature conditions.
However, ionic concentration measurements for clinical examinations and the like are now being performed under increasingly harsh conditions. In addition to the ability to perform measurements precisely, these sensors must have characteristics that (1) enable ionic concentration measurements to be performed continuously and (2) enable ionic concentration measurements to be performed accurately even if there is a sudden change in the temperature of a liquid specimen.
Since the conventional glass membrane electrode requires washing within 30 minutes at most for the reasons set forth above, it is difficult to use the electrode for continuous ionic concentration measurement in a closed system. Though the polymeric membrane electrode, as an alternative to the glass membrane electrode, has been studied to some degree, with relation to the temperature characteristics of the electrode at equilibrium temperature, transient phenomenon accompanying temperature changes have hardly been investigated. This is because it is difficult to measure temperature distribution due to the structure of such an electrode.
Membrane-coated solid electrodes can be made very small. They also do not have an internal liquid chamber and, hence, there is no risk of an internal liquid leaking and contaminating a specimen undergoing measurement. For these reasons, electrodes of this type are attracting considerable interest since they are well-suited for use as clinical sensors.
One requirement that must be satisfied before a membrane-coated solid electrode can be used practically is a high speed of response. In general, response speed tends to vary in inverse proportion to membrane thickness. With a large membrane thickness, an ion carrier membrane-coated solid electrode, in which an ion carrier substance is contained in a polyvinyl chloride (PVC) polymer, exhibits an electrode membrane resistance having a high value of 100 M.OMEGA. or more. If a thin ion carrier membrane is adopted, however, sensor characteristics deteriorate in a short period of time, rendering the electrode unsuitable for practical use. Accordingly, the ideal membrane thickness is on the order of 0.8-1.2 mm, in which case the membrane resistance will be 70-100 M.OMEGA.. Since membrane resistance approximately doubles when the temperature drops by 10.degree. C., problems are encountered when taking measurements at low temperatures. The inventor has found that when the electrode membrane resistance exceeds 50 M.OMEGA., the amount of noise increases and it is difficult to measure the electromotive force of the ion sensor accurately.
Though one method of reducing membrane resistance is to enlarge the electrode area, a greater electrode area necessarily results in a larger electrode. However, an ion small sensor is essential when considering such clinical examination requirements as (1) the ability to measure trace amounts of a liquid specimen, (2) a sensitive reaction to changes in temperature, and (3) the ability to directly measure ionic concentration in body fluids using a catheter or the like. A small sensor has an electrode with a low thermal capacity and attains temperature equilibrium quickly even if there is a sudden change in the temperature of the liquid specimen. Such a sensor is considered to be highly sensitive to temperature changes.